The present invention relates to a scanning optical system used in an optical scanning unit of a laser beam printer or the like.
In a scanning optical system, a rotating polygon mirror is used to scan a light beam from a light source. Conventionally, the polygon mirror is enclosed by a cover unit in order to prevent the generation of sound due to the rotation of the polygon mirror and to avoid damage to the reflecting surfaces of the polygon mirror from collisions with dust or debris in the air. In order to allow the passage of an incident light beam from the light source to the polygon mirror and of a scanning light beam reflected from the polygon mirror, the cover unit is provided with a cover glass plate through which the incident light beam and the scanning light beam pass.
If the cover glass plate is parallel to the rotation axis of the polygon mirror, that is, if the cover glass plate is perpendicular to a main scanning plane, a part of the incident light beam or the scanning light beam may be reflected at a surface of the cover glass plate. This "ghost light" may remain reach the object surface and generate noise in the scanning light beam. This problem is particularly noticeable in an optical system in which one cover glass plate is provided for the transmission of both the incident light beam and the scanning light beam.
The problem of "ghost light" was addressed in Japanese Laid Open Patent Publication No. Sho 63-49726, disclosing a scanning optical system provided with a cover glass plate. The cover glass plate transmits both the incident light beam and the scanning light beam and is inclined with respect to the rotation axis of the polygon mirror. Since the rotation axis of the polygon mirror is perpendicular to the main scanning plane, a normal with respect to the cover glass plate is inclined with respect to the main scanning plane. In this way, any unwanted reflected light, "ghost light", is directed away from the path of the scanning light beam (i.e., away from the object surface).
However, with the cover glass plate arranged such that the normal thereof is inclined with respect to the main scanning plane, the scanning light beam is shifted in the auxiliary scanning direction during transmission through the cover glass plate. As the polygon mirror rotates, the incident angle of the scanning light beam on the cover glass plate in relation to an auxiliary scanning direction varies, and thus, the amount of the shift of the scanning light beam varies throughout the scanning angle, and thus the scanning line formed on the object surface will be curved. That is, there will be a bow in the scanning line.